1. Field of the Invention
The present invention relates generally to an improved design and construction technique for fiber optic hydrophones and hydrophone arrays. More specifically, the present invention comprises a fiber optic hydrophone that has a continuous solid, yet compliant, elastomer core. In some embodiments, plastic microspheres have been added to the elastomer core to provide increased acoustic compliance.
2. Description of Related Art
There are many occasions when it is necessary to detect acoustic signals in an underwater environment. For example, geologic exploration is often carried out by setting small explosives below the ocean's, or other body of water's, surface, detonating the explosives, and then detecting the resulting acoustic signals to determine the structure of various features on or under the sea floor. Additionally, there is a need to be able to detect acoustic signals, such as the sounds emitted by ships, submarines, fish or other animals, that are transmitted under water.
Generally, where acoustic signals need to be sensed or detected in an underwater environment, sensors called hydrophones are used. In many instances, multiple hydrophones are joined together with a specified spacing between each hydrophone to form an array of hydrophones. Such arrays of multiple hydrophones are particularly useful compared to use of single hydrophones where it is necessary to determine the direction the acoustic signals are coming from, or to provide increased sensitivity so as to improve the likelihood of detecting faint acoustic signals.
Conventional hydrophone arrays consist of a series of many piezoelectric elements, or sensors, each of which produces a voltage proportional to the intensity of acoustic signals incident upon the hydrophone. Typical hydrophones available for use in such arrays at present have various circuitry or other electronics associated with the sensor elements located at each sensor in hydrophone. These associated circuits are used for amplification, filtering, digitization, multiplexing and the like of the signals produced by the piezoelectric sensors. Because these additional circuits or electronics are necessarily located underwater, the circuits and electronics are exposed to harsh conditions, such as extreme pressure due to the depth the hydrophone is deployed, or water leakage into the hydrophone housing. To protect the circuits, hydrophones typically include hermetically sealed armored housings. If the circuitry does fail, however, repair of the hydrophone requires that the hydrophone, or, as is typically the case, an entire array or portion of an array, may need to be retrieved from its underwater deployment for diagnosis and repair. Because such repairs are costly and time consuming, and may require the use of specialized vessels and equipment to retrieve the damaged hydrophones, there is a need for a more robust acoustic sensor.
One system that provides improved robustness and reliability uses fiber optic sensors as the sensor element in the hydrophone. Such fiber optic sensors typically use optical fiber wrapped in a high precision winding pattern around compliant, air-backed mandrels as the sensing medium. This arrangement is advantageous in that no additional circuitry or electronics are required at the location of the sensor, making the fiber optic sensors inherently more reliable than other types of conventional hydrophones used in hydrophone arrays.
In a fiber optic sensor, light is sent from a source located in a relatively benign environment through the optical fiber to the sensor. Acoustic pressure waves present in the water dynamically strain the fiber, resulting in a shift in the phase of the light transmitted in the optical fiber. The phase shifted light is compared to a reference signal, creating an interference pattern. The resulting light signal is then sent to an interrogator, which converts the light to an electrical signal for demodulation.
There are several shortcomings associated with presently available hydrophone arrays that use fiber optic sensors as the sensor element. One disadvantage of presently available fiber optic sensors is that the fiber generally is wrapped around discrete, hollow mandrels that are stiff enough to withstand the hydrostatic pressure requirements of deploying hydrophones under water, yet are compliant enough so that the acoustic pressure waves in the water can dynamically deform the mandrel, thereby straining the optical fiber resulting in a phase shift of the light transmitted through the fiber. Accordingly, the mandrels must be formed into sealed, relatively hard plastic or thin metal hollow cylinders that are leak proof against water under the required hydrostatic pressure. In general the mandrels used in presently available fiber optic sensors are stiff and unbendable. This is disadvantageous in that it is useful to be able to manufacture fiber optic sensor arrays in long continuous sections, and to store such arrays on circular drums, from which the fiber optic sensor array may be deployed and retrieved, and such long sections need to be flexible.
To facilitate the flexibility needed to store the fiber optic sensor arrays on a circular drum, the mandrels must be made into short cylindrical pressure vessels, such as, for example, capped tubes, with flexible links between the capped tubes to form long continuous bendable sections. A considerable amount of labor must be used to assemble the optic fiber wrapped on the air-backed mandrels into interferometers. This laborious process includes preparing the optical fiber, splicing and recoating the optical fiber, dressing the optical fiber, mechanically assembling the pressure vessels, and sealing and testing the vessels and optical fiber to ensure that the resulting assembly is water tight and functions as desired.
As fiber is wound around the long continuous, bendable sections, great care must be exercised to ensure that the mandrel/flexible link interfaces do not have any sharp or uneven surfaces, and do not separate, shift, or deform under pressure or tension, which will break the optical fiber. In addition, array strength members and extra optical fibers often must be placed along the outer surface of the continuous section, leading to optical fiber damage during reeling/unreeling operations as a result of friction, bending, crushing, and the like.
An additional shortcoming with air-backed mandrel optical fiber hydrophones is that such devices have a flat optical phase response to acoustic input as a function of acoustic frequency. This is disadvantageous in acoustic frequency ranges that contain unwanted acoustic signals, such as noise caused by fish, whales or other sound source, whose presence limits the dynamic range of the overall system unless very high sample rates are used by the system electronics to interrogate the sensors to allow signal analysis and canceling of the noise.
The spaces between and around the mandrels used for the fiber optic sensors may also be filled with a liquid in an attempt to provide improved acoustic coupling and thus sensor sensitivity as well as to improve or control the buoyancy of the array. Such construction may be disadvantageous because such liquid-filled fiber optic sensor arrays are susceptible to puncture during deployment and reeling operations as well as during normal operation, and leakage of the fluid typically results in failure of the array. Moreover, where a fluid such as kerosene or kerosene-like liquids or other possibly environmentally hazardous material is used, leakage of the fluid can contribute to unwanted pollution.
What has been needed, and heretofore unavailable, is a reliable, robust fiber optic acoustic sensor that has eliminates the disadvantages of air-backed or fluid filled arrays, yet provides for increased sensitivity and ease of manufacture. The present invention satisfies these and other needs.